1. Field of the Invention
This invention relates to the manufacture of galvanic cells. In particular, it relates to the manufacture of negative lead acid plates and a process for drying such plates.
2. Description of the Prior Art
It has long been known that a wet electrochemically formed negative plate for a lead acid storage cell will oxidize when exposed to air. A battery built of such plates must be charged before it will deliver an electric current. If the negative plate is dried in such a way that it is not oxidized, a dry battery can be built which will deliver electric currents as soon as it is filled with electrolyte This has been found to be desirable in the storage battery market place.
The active material of a formed negative plate is lead sponge. It has a very large surface and is very active chemically. When a wet plate is dried in an atmosphere of air, it passes through a stage where some although not all the water has been removed from its pores. When the moisture content is at a critical value, some of the lead sponge becomes very reactive toward atmospheric air. The reaction produces heat, drying other parts of the sponge into the reactive state. This starts a chain reaction which continues until the entire mass of sponge has reacted and has been changed from metallic lead sponge to lead oxide. The heat evolution is considerable. Piles of plates will often get hot enough to melt -- which of course completely destroys them. If plates are dried without passing through the critical point of moisture, oxygen and temperature build up, they can be completely dried without the run-away oxidation. The lead sponge, when fully dry, is not very reactive to air at room temperature so that when once dried they can be stored for long periods of time.
During the drying period, the plates are unreactive to such gasses as steam, nitrogen and carbon dioxide (although at high temperatures somewhat above 350.degree.F. (177.degree.C.) the lead sponge may react with carbon dioxide in the presence of moisture to form lead carbonate). It is also unreactive to the inert gasses of the zero group of the periodic table as well as countless organic vapors. Because of exposure, it is desirable to use the least costly of the gasses noted above for drying battery plates. The several gasses suggested above and including steam, nitrogen and carbon dioxide can be considered to be inert to lead sponge during a drying cycle.
Many processes have been tried and used for drying negative plates free from or with only slight oxidation. Among such processes have been vacuum drying, drying in hot oil, drying between hot plattens, drying in steam and drying in inert atmospheres and numerous other methods have been demonstrated. It is possible to dry plates by exposure to a blast of dry air with only moderate oxidation (10% or so). The very best processing such as vacuum drying has given plates having 1/2% or less oxide. In general, an oxide content between 5 and 10% is satisfactory for many battery types. Storage battery plates dried between hot plattens have been cooled by being inserted in a box containing dry ice (solid carbon dioxide). This treatment cools the plates very rapidly and at the same time protects them from oxygen.